Various methods have been devised for enabling the safe continued operation of unpressurized or underpressurized tires without damaging the tire further and without causing poor steering and vehicle handling, over a distance from the place where the tire lost its pressure to a place desired by the driver, such as a service station where the tire can be repaired or changed. Loss of tire pressure can result from a variety of causes such as puncture of the tire by a foreign object such as a nail or other sharp object piercing the pneumatic tire installed on a vehicle.
Pneumatic tires designed for continued operation under conditions of loss of pressurization or underpressuration are referred to as extended mobility technology tires or EMT tires. They are also called runflat tires. EMT or runflat tires are designed to be driven in the uninflated condition, whereas the conventional pneumatic tire collapses upon itself when subjected to a vehicle load while uninflated. The sidewalls and internal surfaces of EMTs do not collapse or buckle onto themselves. In general, the terms “EMT” and “runflat” mean that the tire structure alone has sufficient strength to support the vehicle load when the tire is operated in the uninflated state. In particular, the sidewalls are reinforced to carry the tire's load without recourse to the use of other supporting structures of devices that are disposed internal to but separate from the tire. An example of the latter internal supporting structure is shown in U.S. Pat. No. 4,059,138, entitled “Run-flat Tire and Hub Therefor.”
Numerous other methods and tire construction have been used to achieve workable runflat tire designs. For example, a runflat tire structural design described in U.S. Pat. No. 4,111,249, entitled the “Banded Tire,” shows the use of a hoop or annular band approximately as wide as the tread placed under the tread. The hoop in combination with the rest of the tire structure could support the vehicle weight in the uninflated condition.
Generally, runflat tires incorporate reinforced sidewall designs of the type mentioned before. Such sidewalls are thicker and stiffer, so that the tire's load can be carried by an uninflated tire without compromising vehicle handling until such reasonable time as the tire can be repaired or replaced. The methods of sidewall stiffening include the incorporation of inserts or fillers generally having, in cross-sectional view, a crescent shape. Such inserts, or wedge inserts as they are often called, are located in the inner peripheral surface of the sidewall portion of the carcass, which is the region in the tire usually having the lowest rigidity. In such runflat or EMT designs, the entire sidewall has an approximately uniform thickness corresponding to the thickness of the bead region, so as to provide runflat supporting rigidity. The sidewalls of such tires, when operated in the uninflated condition, experience a net compressive load, though the outer portions of the sidewalls are necessarily in tension due to the bending stresses while the insides are correspondingly in compression, especially in the regions of the sidewall that are in the midway point between the tire's bead region and that portion of the tread that is most immediately adjacent to the ground-contacting portion of the tread.
Due to the large amounts of rubber required to stiffen and reinforce the sidewalls, heat buildup as a result of cyclical flexure of the sidewalls is a major factor in tire failure, especially when the uninflated tire is operated for prolonged periods of time and at high speeds. During normal-inflated operation, the hysteresis of the material of the thickened sidewalls contributes to the tire's rolling resistance, which reduces the vehicles fuel mileage. The additional weight of the wedge insert reinforcements also presents a challenge to the designer of runflat tires.
U.S. Pat. No. 5,368,082 ('082), having a common assignee with the present invention, disclosed the first commercially accepted runflat pneumatic radial ply tire. The '082 patent describes the employment of sidewall wedge-insert reinforcements to improve stiffness. Approximately six additional pounds of weight per tire was required to support an 800 lb load in this uninflated tire. This earlier invention, although superior to prior attempts at runflat tire design, still imposed a weight penalty that could, however, be offset by the elimination of a spare tire and the tire jack. However, this weight penalty was even more problematic when the engineers attempted to build high-aspect-ratio tires for large, heavy vehicles, such as touring sedans. The required supported weight for an uninflated luxury car tire can exceed 1,000 lbs. These taller sidewalled tires, having aspect ratios in the 55% to 65% range or greater, means that the sidewall bending stresses are several times that of the earlier low-aspect-ratio runflat tires. The ability to handle such loads meant that the sidewalls and overall tire had to be stiffened to the point of adversely influencing riding comfort and some handling characteristics. Accordingly, EMT or runflat tire design requires that there be no loss in riding comfort or vehicle handling. In the very stiff suspension performance type vehicle, such as sports cars and various sport/utility vehicles, the ability to provide such runflat tires was relatively straightforward compared to providing similar runflat tires for luxury sedans which require a softer ride. Light truck and sport utility vehicles, although not as sensitive to ride performance, provide a runflat tire market that ranges from accepting a stiffer ride to demanding the softer luxury type ride.
Thus the EMT or runflat tire designs incorporating sidewall wedge-insert reinforcements of the sort described in the '082 patent add weight to the tire while also causing flexural heat buildup in the wedge insert material, especially during runflat operation when the magnitude of the cyclical sidewall flexure is greatest. And, as mentioned, normal-inflated riding comfort is also compromised by the additional sidewall stiffness, and the tire's rolling resistance is greater than that of corresponding non-runflat designs. Thus the design goals of EMT or runflat tire designers are to minimize tire weight, minimize the potential for heat buildup during runflat operation (especially at high speed), and, during normal-inflated operation, give minimum rolling resistance, good riding comfort and acceptable handling characteristics.
A tire's service life is dominated, of course, by full-inflated operation. Therefore, the main or most immediate above-listed design goals are good riding comfort and low rolling resistance, with tire weight being of secondary importance to the extent that it should not adversely affect the performance of sports-type vehicles. As for heat buildup, it is mostly a problem during runflat operation, being a major contributing factor to the inevitable deterioration of the tire when operated without inflation.
Another example of an EMT or runflat tire design that at least part achieves the same riding-comfort problem while also addressing the tire weight problem is covered in Patent Application Serial No. PCT/US98/13929, entitled IMPROVED SIDEWALL WITH INSERT CONSTRUCTION FOR RUNFLAT TIRE, and having a common assignee with the present invention. In this invention, a metal-reinforced first ply carries a major part of the compressive load during runflat operation, which allows the thickness of the wedge-insert reinforcements to be less than otherwise would be needed. That metal-reinforced first ply experiences primarily tensile loading during normal-inflated operation, and otherwise provides improved sidewall flexibility during normal inflated operation. During runflat operation, however, the metal members of the first ply undergo substantial compressive loading, especially in the sidewall portions that are most immediately adjacent to the ground-contacting portion of the tread. The tire of the invention described by the PCT/US98/13929 application thus addresses the design goals of full-inflated riding comfort, tire weight and extended runflat service life.
Another example of a tire designed to be usable without normal inflation pressure is found in Great Britain Patent Specification No. 867,103, published May 3, 1961, which discloses a tire having a stiffener made of rubber and attached to the inside of each sidewall. The stiffener extends circumferentially, continuously all the way round the sidewall of the tire, and extends radially from a point near the radially inner extremity of the tire to a point near the center of the tread. The stiffener is divided into individual circumferential ribs, separated from each other by wedge-shaped gaps.